The invention relates generally to multiple output flyback DC/DC converters and more specifically to a method and an agement for regulating low output voltages to tight tolerances in multiple output flyback DC/DC converters.
There is a demand for converters supplying in addition to a main output DC voltage, at least one additional output DC voltage lower than the main output voltage.
If diodes are used for generating the additional output DC voltages, the voltage drop across the diodes can be as high as 0.5 V. This means that for low output voltages, e.g. xe2x89xa63.3 V, the efficiency will be low.
When high efficiency is needed, synchronized rectification has to be used since the voltage drop across synchronous rectifiers, i.e. MOSFETs, is much lower than across diodes.
In multiple output converters, the main output voltage is regulated in a manner known per se by regulating the duty cycle of the primary switch. However, additional output voltages can not be regulated in multiple output converters by regulating the duty cycle of the primary switch but have to be separately regulated, e.g. by means of complicated switched series-regulators.
The object of the invention is to bring about a method and an arrangement for controlling a MOSFET used as synchronous rectifier to regulate low output voltages to tight tolerances.
This is attained in accordance with the invention by controlling the switch-off time of the MOSFET to vary its on-period.
Hereby, the output voltage can be controlled between the voltage drop in the body diode of the MOSFET and the saturation voltage of the MOSFET. This voltage difference is only about 0.3 to 0.4 V but large enough for regulating a 3.3 V output voltage to tight tolerances.